Heretofore, it has been proposed to utilize a flywheel in a bicycle for storing energy imparted to the bicycle upon going downhill for later propulation aid during uphill travel. While various bicycle designs have been proposed in the art none are conducive for use with an extremely high angular velocity flywheel. For example, the placement of the flywheel in prior designs was at relatively weak portions of the bicycle frame which also would expose the flywheel to accidental touching. Further, the prior art has not pointed out how to mechanize an appropriate slip clutch mechanism to enable selective engagement of the flywheel for propulsion aid or braking.